Infinite Wish Loop
"I will grant you three wishes, but no wishing for more wishes."
"Gosh, I wish I could wish for more wishes."
Role-playing games with powerful wizards usually have a spell called "Wish" or similar at the top tier of spellcasting, and it's supposed to be the wildcard of do-as-thou-wilt magical effect. But any DM knows the problem with giving players a blank check: they're going to actually try to cash it in. If the game wasn't written by a noob, the spell description for the Wish spell will have specific limits on what can be wished for, including "No wishing for more wishes!" and "No wishing for more beings who grant you wishes" in order to prevent cheesing via infinite wish loops.
...Ha ha, screw that. The Wish spell is supposed to be an "I win" button, and we're going to win the fuck out of this game. This page should be a list of how to get nigh-limitless uses of the Wish-type spell effect in games.
Candle of Invocation Loop
This is for D&D 3e. Get a Candle of Invocation (8,400gp, could show up in the treasure for encounter level 12). If the Candle matches your alignment, you can use its gate feature. The Candle has caster level (CL) 17, so you can use the Candle to gate in 34 hitdice of creatures, and have them serve you immediately for 17 rounds without needing payment. This will destroy the candle, but no worries.
Gate in an efreet (10 hitdice). Among its spell-like abilities is "1/day grant up to three wishes (to non-genies only)". Wish one: Ale. Wish two: Whores. Wish three: another Candle of Invocation, pretty please.
Cheese aficionados should recognize this method as a crucial step on the Path to Pun-Pun.
D&D with the Eberron setting. Your character has the 'Wand Surge' feat, and you've acquired a Wand of Unfettered Heroism with at least one charge left. Then get yourself a Staff of Wishes with at least one charge left.
Zap yourself with the wand, which gives you a free action point every round for one minute. Use that action point every round with the Wand Surge feat to use a magic item without expending a charge. One minute = 10 rounds, so that's ten uses of the Staff of Wishes without expending the last charge.
Chain of Simulacra
The exploit of choice in 5th edition D&D involves the Simulacrum spell, which allows you to create a clone of yourself (or any other character) that has the original's spells and spell slots. Including Wish. This lets players get around that pesky "Casting Wish means you have a 1/3 chance of never being able to use it again" restriction, because it isn't the player who is casting the spell. This is also used to create an ever-expanding army of 17th level spellcasters, as the Simulacrum can cast its one use of Wish (simulacrum spell slots don't regenerate) to make another Simulacrum, and that one spends its slot to make another, until... until nothing, because this train will not stop, and has no reason to stop. You won D&D, good job. I hope you're happy.
This can also be used to implement the Peasant Railgun, if you really wanted to.